This invention relates to interface circuits for operation between a controlling device and a controlled device with optical couplers for selective application of function initiating signals to the controlled device and for status indicating signals from the controlled device to the controlling device. The invention has particular utility in solid state power controller apparatus.
Solid state power controllers of generally known configuration are used for controlling conduction in a load circuit between a power source and a load in response to low level control signals from a location that is perhaps remote from the load circuit. For example, in aircraft power systems, power controllers near their associated loads may be controlled from the aircraft cockpit by a controlling device such as a computer controlled multiplex system. Among the required functional capabilities of such apparatus is that there be electrical isolation between the controlling device and the controlled device both for the signals that initiate functions in the controlled device and for the signals indicating the status of the controlled device that are generated in it and supplied back to the controlling device. For these purposes a control interface circuit is used.
It has been a practice to use optical couplers as the isolation devices in control interface circuits. An optical coupler device comprises a light emitting element such as a light emitting diode emissive of either visible or invisible light, that is radiatively coupled with a light sensitive element such as a photodiode or a phototransistor. The multiplicity of required functions can be provided by having in the interface circuit one such optical coupler for applying the function initiating signals to the controlled device and one or more other optical couplers for translation back to the controlling device of status indicating signals. The optical couplers are associated with impedances in series relation with the primary optical coupler so that the conduction of the additional optical coupler devices modifies the amount of impedance in the circuit between the control terminals and thus can vary the voltage at the control terminals.
Speed requirements play a significant role in the design of a suitably effective interface circuit. The light sensitive element at the input of the controlled device is normally associated with a time delay circuit so that the controlled device only responds by changing its conductive state upon application of a control input signal of a given duration. This is useful in that it permits an arrangement in which the controlled device only responds to a steady input signal while the status indicating signals from the controlled device, such as that indicating a fault condition in the load circuit, fed back to the controlling device, may result in the application of only brief sampling pulses insufficient to turn on the controlled device. In general, the control interface circuit must promptly and accurately indicate the value of effective resistance between its terminals because the controlling device measures the resulting voltage and thereby ascertains the status of the remote device. It has been found that currently available designs for such control interface exhibit response times that are undesirably slow and do not provide adequate operation in use with control signals of short sampling time such as about 50 microseconds. Brief sampling pulses are desirable because the computer controlled multiplex system is usually associated with numerous power controllers by time sharing. General background information of the use of multiplex systems for controlling power controllers may be found in Geyer, et al. U.S. Pat. No. 3,842,249, Oct. 15, 1974. Prior control interface circuits employing optical couplers are described in Perkins, et al., "Power Controller Breadboard and Development Requirements", Report 2-57110/4R-3195 for the Department of the Navy, March 1975, and Billings, "Aerospace Technology Development of Solid Stage Remote Power Controllers . . . ", Report No. NASA CR-135199 for the National Aeronautics and Space Administration, June 1977.
It has been discovered that response times of control interface circuits may be greatly improved in accordance with the present invention. The improvement resides in the recognition that the primary cause of slow response is the parasitic capacitance of the optical isolator device. This capacitance causes a built-in time delay to occur so that when a sampling pulse is applied, current flows through this parasitic capacitance into the associated transistor switch causing it to partially turn on and until this capacitance is essentially fully charged, the circuit does not indicate the appropriate resistance at its control terminals. It has been found with the commercially available components over a desired temperature range such as about -55.degree. C. to 125.degree. C. that the response time can range up to several times a desirable sampling period of about 50 microseconds.
The invention provides an appropriate arrangement of an optical coupler associated with a series string of impedance elements in the control interface circuit with those impedance elements selectively switched in or out of the effective circuit between the control terminals by transistor switches controlled by optical coupler elements that are arranged in association with the control input terminals so that their operating voltage is supplied from the control input terminals but is associated with means for limiting the operating voltage to a magnitude that results in a full charge on the parasitic capacitance of the optical couplers within a time that is short in relation to the sampling pulse time of the controlling device.
It has been found that circuits can be arranged to perform within a period of approximately 2 to 15 microseconds which adequately permits the operation of the system with sampling pulses of about 50 microseconds.